1. Field of the Invention
The present invention relates to call controlling in a mobile communication system, particularly to an apparatus and a method for performing a call control in a mobile communication system, which can offer an optimum service economically and efficiently.
In a mobile communication system, a visitor location register is used to store subscriber data required for offering various services, including information on a location of a base station (or mobile switching center) in the coverage area of which a mobile station resides (hereinafter simply called mobile station location) and information for identifying the subscriber.
Accordingly, even when the storage capacity of the visitor location register is not sufficient enough, call controlling apparatus and method which can process various requests for services (e.g., location registration, call originating, call incoming) to/from a portable telephone are in great demand.
2. Description of the Related Art
FIG. 1 is a block diagram showing principal devices of a conventional mobile communication system, in which an exchange is connected to a mobile station (abbreviated to MS) via a base station (not shown). The mobile station is a telephone which changes from location to location, such as a car telephone and a portable telephone.
exchange, which is comprised of a mobile switching center (abbreviated to MSC) 1A and a visitor location register (abbreviated to VLR) 2A, conducts a location registration for the mobile station on receiving a location registration request signal therefrom. It also establishes a connection between mobile stations by sending a call incoming request signal to a called mobile station on receiving a call originating request signal from a calling mobile station.
The MSC 1A is comprised of a call controller (abbreviated to Call Cont in the figure) 11a and an MISC memory 12a. The MSC memory 12a has areas called call connection information areas (abbreviated to CIA in the figure) for a predetermined number of subscribers. The area stores therein call connection information required to connect a call, including the information on a base station, connection path, billing and routing. The call controller 11a controls so as to connect the call based on the information stored in the MSC memory 12a. 
The VLR 2A is comprised of a call controller (abbreviated to Call Cont in the figure) 21a and a VLR memory 22a. The VLR memory 22a has a subscriber data area for.storing roaming subscriber""s information, including the mobile station location, subscriber identifier (e.g., telephone number, telephone set number), authentication, restriction indicator, supplementary service and network identity, for a predetermined number of subscribers. The call controller 21a manages the subscriber data of a roaming subscriber in the VLR memory 22a. 
A home location register (hereinafter called HLR) 3A is composed of a call controller 31a and an HLR memory 32a. The HLR memory 32a stores therein subscriber data for home subscribers, including the mobile station location, subscriber identifier (e.g., telephone number, telephone set number), authentication, restriction indicator, supplementary service. A call controller 31a manages subscriber data of the home subscribers in the HLR memory 32a. 
When the location registration request signal for a mobile station is received from the MSC 1A, the VLR 2A obtains a subscriber data area (abbreviated to SDA in the figure) for storing the subscriber data in the VLR memory 22a. Then, it retrieves the subscriber data from the HLR 3A and stores and holds the retrieved subscriber data in the area while the mobile station resides in the network concerned. When receiving a call originating request signal from a mobile station or when sending a call incoming request signal to a mobile station, the VLR 2A reads the subscriber data from the area to perform a call connection.
FIG. 2 is a flowchart illustrating a conventional location registration, which is performed by the VLR 2A.
(1) When receiving from a mobile station (MS) a location registration request signal including the mobile station location information, the MSC 1A requests the VLR 2A to register the location.
(2) The VLR 2A attempts to obtain the subscriber data area in the VLR memory 22a. 
(3) When the attempt is successful, the VLR 2A requests the HLR 3A to reply with the subscriber data of the mobile station.
(4) The VLR 2A stores the replied subscriber data in the obtained subscriber data area and thereafter, informs the MSC 1A of completion of the location registration (hereinafter, the operation of steps (3) and (4) is called normal processing of the location registration).
(5) If the attempt in step (2) is unsuccessful, the VLR 2A indicates the MSC 1A to disconnect the call.
According to the conventional method as described above, it was a problem that when there was no subscriber data area left in the VLR memory 22a, the VLR 2A had to reject all the service requests for location registration, call originating, call incoming, etc., from/to a roaming subscriber.
It was another problem that increasing the storage capacity of the VLR memory 22a to overcome the above problem increases the cost of the VLR 2A and therefore the cost of the system.
It is an object of the present invention to provide an apparatus and a method for performing a call control in a mobile communication system, which can offer an optimum service to the mobile station efficiently without rejecting the service requests from/to a roaming subscriber.
It is another object of the present invention to provide an apparatus and a method for performing a call control in a mobile communication system, which can offer an optimum service economically with a small amount of storage capacity of the visitor location register memory.
To achieve the above and other objects, the present invention provides determination means and storing means. In an apparatus for performing a call control in a mobile communication system including a first storage storing subscriber data for a home subscriber, a second storage storing call connection information and a third storage for storing subscriber data for a roaming subscriber, the determination means determines in response to a service request signal, that subscriber data is to be stored in the third storage and that there is no area therein for storing the subscriber data. Based on the determining by the determination means, the storing means retrieves the subscriber data from the first storage and stores the retrieved data in a second storage area. Also, the present invention provides the aforesaid determination means and overwrite means. Based on the determining by the determination means, the overwrite means retrieves the subscriber data from the first storage and overwrites the retrieved data in a third storage area.